The present invention relates to the extrusion of thermoplastic resinous materials and in particular granular thermoplastic resinous materials which are admixed with additive amounts of fine powdery materials. Specifically, such admixtures may be encountered in the extrusion of thermoplastic foams such as polystyrene foam. In general, in a direct injection process, polystyrene foam is produced by feeding granular polystyrene resin pellets to the feed hopper of an extrusion apparatus where the polystyrene granules are thereupon fed into the extruder proper. The continuously rotating extruder screw forwards the polystyrene granules as they are heated and melted to form a molten mass which continues to be forwarded by the screw. At some point after the polystyrene has been completely melted, a blowing agent such as, for example, a freon, pentane, isopentane or the like is introduced through the extruder barrel and is thoroughly admixed with the molten polystyrene resin. Finally, the mixture of blowing agent and molten polystyrene is expressed through a die orifice whereupon the sudden reduction in pressure causes the gasification of the major portion of the blowing agent which results in the formation of a foam structure upon cooling. It has been found in the prior art that, in order to control the cell size of the individual cells contained in the polystyrene foam, a cell size control additive must be added to the polystyrene blowing agent mixture before it is extruded. Cell size control is important since the size and uniformity of the foam cells determines the physical characteristics of the final foam product, e.g. stiffness, toughness, brittleness, flexibility, etc. It has been found that when materials such as talc, silica, mixtures of citric acid and sodium bicarbonate, and other materials are added in precisely controlled amounts and, in the case of a 2-component nucleating system, in stoichiometric ratios, the size of the cells in the final foam produce may be controlled within defined limits. Further, it has been found that it is desirable that such cell size control additive materials be admixed with the polystyrene resin pellets prior to their introduction into the barrel of the extruder. In a conventional extrusion system the point of introduction of the resin and cell size control agent is at the feed hopper of the extruder. This point of introduction is immediately adjacent to the extruder throat, i.e. that portion of the extruder screw and barrel beyond which the shank end of the screw extends out of the extruder barrel and into engagement with the extruder screw drive and thrust mechanism. A suitable seal must surround that portion of the extruder screw shank intermediate the hopper zone and the end of the extruder barrel to insure that leakage of resin and, in particular, fine powdery additives out of the back end of the extruder does not occur. Additionally, return flights are machined into the shank of the extruder screw in this seal area to insure that material which does leak back between the screw shank and barrel is positively returned by the forwarding action of these return flights. Obviously, a defective or inadequate sealing of the clearance space surrounding the screw shank at the point where it leaves the extruder barrel will result in an undesirable leakage of the fine, powdery cell size control additive through this clearance and out of the extruder. Such leakage of cell size control additive powder makes it difficult, if not impossible, to maintain a precise amount of cell size control additive in the extrusion system. As hereinbefore noted, precisely controlled amounts of such additive materials are necessary in the extrusion system to control the individual cell size in the final foam product within a particular defined range of cell size. Hence, loss of random amounts by leakage of the additive material from the extruder proper through ineffective or defective seal arrangements inhibits, or makes impossible, effective cell size control in the final foam product.
When designing a seal arrangement for an extruder apparatus, factors which must be taken into consideration include the temperature and pressure conditions to which the seal will be exposed, rotational velocity of the extruder screw, the medium which is to be sealed, and the extruder screw run out. These are basic considerations for rotary seal design. In the case of foam extrusion employing a fine powdery cell size control additive, the sealing problem is complicated with frictional heat, rolling pressure, the medium, i.e. the powder being sealed, and the rotational run out of the screw. By rotational run out of the screw is meant that the screw shank portion in the area of the extruder throat being sealed does not rotate around its geometrical center. This results in a given point on the circumference of the screw shaft circumscribing a path greater than the screw diameter as the screw rotates, resulting in an up-and-down and side-to-side motion of the turning screw shank. This run out is primarily caused by forces which act on the screw in the screw thrust and drive mechanism. For example, spaced apart sets of bearings which support the screw in the drive area may be slightly misaligned. This misalignment causes the screw shank to rotate about an axis which is offset from the true geometrical center of the screw resulting in eccentric rotation of the screw. A rubber seal surrounding the screw shank in the extruder throat might flex to accommodate this rotational run out of the screw and remain in continuous sealing relationship around the periphery of the screw, but sealing materials such as rubber require lubrication and cooling. Moreover, the nature of the material being sealed, i.e. a fine powdery material, when admixed with a lubricant such as oil, which is present in the seal area, forms an abrasive paste which causes errosion and subsequent failure of such a rubber seal. Another alternative would be to employ, in place of a rubber seal, a bronze bushing which would surround the screw in the area it is desired to seal. Such bronze bushings are conventionally fixed in place and, therefore, the rotational run out of the screw, hereinbefore described, would cause errosion and abrasion on the inner surface of the bushing resulting in the formation of a gap between the bushing and the rotating screw allowing the powdery additive material to leak through the gap, past the seal, and out of the extruder at a high rate.